FIGS. 5(a)-5(d) are explanatory views showing various types of flaws which develop in a mother tube manufactured by rolling a hollow shell using a mandrel mill.
FIG. 5(a) shows inner surface indentation flaws which are indentations 4 in the inner surface of a mother tube P. FIG. 5(b) shows perforation flaws which are holes 5 occurring when inner surface indentation flaws advance and reach the outer surface of a mother tube P. FIG. 5(c) and FIG. 5(d), which is a cross section in the circumferential direction of the mother tube P of FIG. 5(c), show a wrinkle flaw which is a portion 6 where the outer surface of a mother tube P is folded inwards. Each of these flaws is a major cause of occurrence of defective mother tubes.
In a mandrel mill, the presence of the above-described various flaws has conventionally been detected by direct visual observation of a rolled mother tube by an operator working in a control room located in the vicinity of the mandrel mill.
However, in recent years, as automation of tube forming facilities progresses, a control room is situated in a location remote from a mandrel mill. Therefore, situations have developed in which an operator cannot directly visually observe various types of flaws in a mother tube after rolling. Accordingly, even if various types of flaws develop in mother tubes which have undergone rolling using a mandrel mill, they cannot be rapidly detected, and there is a possibility of a larger number of defective products developing than in the past.
For example, Patent Documents 1-6 disclose inventions in which in order to suppress variations in wall thickness of the end portions of a mother tube which is rolled using a mandrel mill and in thickness deviations in the circumferential direction of the mother tube, the wall thickness of a mother tube rolled in the mandrel mill is measured by a wall thickness gauge positioned on the exit side of the mandrel mill, and based on the results of measurement, the rolling conditions of the mandrel mill are suitably changed.
Patent Document 1: JP H7-246414 A1
Patent Document 2: JP H8-71616 A1
Patent Document 3: JP 2001-293503 A1
Patent Document 4: JP 2002-35817 A1
Patent Document 5: JP 2003-220403 A1
Patent Document 6: JP 2004-337941 A1